This invention relates to pumps having isolation valves and more particularly to pumps having integral diffuser-isolation valves.
In a nuclear reactor power plant, the nuclear reactor generates heat which is transferred from the reactor coolant to a location remote from the nuclear reactor for the generation of steam and electricity in a conventional manner. In order to circulate the reactor coolant through the system, reactor coolant pumps are located in the reactor coolant circulatory system for pumping the reactor coolant through the system. Typically these reactor coolant pumps comprise a vertical pump with an electric motor vertically mounted on the pump by means of a motor stand. The rotation of the motor's drive shaft causes the pump's shaft to rotate, which causes the pump to circulate the reactor coolant through the reactor system in a manner well-known to those skilled in the art.
In many commonly known nuclear steam supply systems, there are a plurality of reactor coolant pumps disposed in fluid communication with the nuclear reactor vessel. This may be accomplished by either a loop-type arrangement or a pool-type arrangement as is commonly understood in the art. During reactor operation, the coolant pumps simultaneously pump reactor coolant through the nuclear reactor core where the reactor coolant passes in heat transfer relationship with the fuel assemblies therein. From this common pool of reactor coolant, the reactor coolant exits the reactor core and passes into the remainder of the reactor system. While the plurality of reactor coolant pumps function cooperatively under normal reactor conditions, under abnormal conditions, the interconnection of the reactor coolant pumps may result in damage to the system.
One such abnormal condition that may result in damage to the system is the failure of one of the coolant pumps while the other coolant pumps remain operating. In this situation, the operating coolant pumps may cause reactor coolant to be conducted through the non-operating pump in the reverse direction of its normal flow. In addition, it is necessary to be able to isolate a particular reactor coolant pump from the remainder of the operating system when the plant is operating at partial load to prevent coolant pressure loss associated with reverse flow through an idle pump. Although prevention of reverse flow through the non-operating coolant pump when the other pumps are operating is important, it is not the only consideration. Another important consideration is that in the event all pumps fail simultaneously such as in an electrical power failure, the path of the reactor coolant must remain open to allow natural circulation of the reactor coolant through the reactor core to facilitate cooling of the reactor core.
One device known to prevent reverse flow when one coolant pump is non-operational and to allow natural circulation when all cooled pumps are non-operational is a type of swing valve that is placed in the piping network between the coolant pump and the reactor core. This type of valve consists of a substantially circular metal flap attached by a hinge arrangement to the inside of a horizontal segment of piping such that under reverse flow the metal flap pivots about the hinge into an acute angle with respect to the hinge, thus blocking the flow path. However, when all coolant pumps are not operating, the metal flap hangs from the hinge in a substantially vertical attitude without contacting the side of the pipe opposite the hinge, thereby allowing natural circulation through the primary loop by allowing coolant to flow between the metal flap and the side of the pipe opposite the hinge because the natural circulatory flow is not sufficient to force the metal flap into the acute angle necessary to block the flow. While this device does solve some of the reverse flow problems, it creates additional problems in that the metal-hinge flap attachment creates a wearing surface and a surface susceptible to self-welding under high temperature coolants which thereby demand frequent maintenance attention.
There are many check valves in the art that allow flow in both directions under appropriate conditions. These check valves generally consist of a float member having a first end manufactured to conform to the shape of a valve seat and having a second end formed in a winged configuration capable of spanning a valve opening, opposite the valve seat, for allowing flow through the valve opening and between the winged configuration. Under normal conditions, a fluid is allowed to flow through the valve by passing through the winged configuration; however, under certain pressure conditions the first end of the flow member is forced against the valve seat thereby preventing flow through the valve. While these valves do perform necessary functions, they are not capable of being positively actuated to selectively prevent flow of reactor coolant therethrough.
Ideally, a reactor coolant pump must have a valve that is capable of selectively shutting off flow in either direction. Furthermore, to provide natural circulation through the reactor system, the valve should introduce a minimal discharge pressure drop when in the open position. Also, the valve should not hinder pump related functions or significantly reduce pump efficiency in its operating range. Therefore, what is needed is a pump having a valve that is capable of selectively isolating the pump from the remainder of the coolant system in a manner that does not reduce the pump efficiency when the pump is operating normally.